The pear-shaped, colorless gem weighs 101.73 carats, and will be up for
It's the first time the "flawless" diamond is being offered for sale, which means whoever buys the gem will also be able to name it.
The stone was cut from a rough diamond found in the Jwaneng mine in Botswana, which is co-owned by De Beers and the government of Botswana. It originally weighed 236 carats, and took 21 months to cut and polish, according to Christie's.
The $20 million price could easily be reached given that the famous Archduke Joseph Dimaond — a similarly colorless 76.02-carat stone — was sold for $21.4 million at Christie's, Geneva back in November 2012.
The most expensive gem ever sold at auction was a 24.78 carat "Fancy Intense Pink" emerald-cut at Sotheby's, Geneva that went for $45.6 million in November 2010.